Previously, some evidence was provided for a possible role of glucagon during eye growth regulation, at least in the chick: Quisqualate-injection causes prominent retinal degeneration but leaves the mechanisms of deprivation myopia intact (Fischer et al., J. Comp. Neurol. 1998;393:1-15).
Among the small number of cells largely unaffected by Quisqualate are dopaminergic and glucagonergic amacrine cells. Moreover, amacrine cells which are immunocytochemically double-stained both by antibodies against glucagon and the immediate early gene ZENK, show a sign of defocus specific up-regulation of ZENK with positive lenses, and down-regulation with negative lenses after 30 min of treatment.
The glucagonergic ZENK-immunoreaktive cells may contribute to the visual regulation of ocular growth (Fischer et al., Nat. Neurosci. 1999;2:706-712). Previously, it was shown that the amount of proglucagon mRNA increases during treatment with positive lenses (Feldkaemper et al., Invest. Opthalmol. Vis. Sci. 2000;41:1623-1628).

Since glucagon is one promising candidate for a messenger that regulates eye growth, the influence of glucagon, a glucagon agonist and an antagonist on lens induced myopia and hyperopia development was investigated.
As a result, the pharmacological experiments supported the hypothesis that glucagon may act as a growth inhibiting signal.

Experiments in our and Bill Stell`s lab (Calgary) supported the suggestion that glucagon contributes to visual regulation of ocular growth at least in the chick.
Glucagon exerts its regulatory effects by binding to the glucagon receptor. We therefore aim to demonstrate the cellular localization and regulation of glucagon receptors in the chick retina using in-situ hybridization studies.
The glucagon receptor belongs to the superfamily of seven transmembrane-spanning receptors that couple to heterotrimeric guanine nucleotide-binding proteins. Binding of glucagon to the receptor leads to a rapid and long-lasting action.
In the glucagon system, modulation of adenylate cyclase occurs, initiating the production of cAMP. Moreover, glucagon can exert effects on signaling pathways via cAMP-independent interactions leading to a stimulation of phospholipase C and the release of Ca2+ from IP3-sensitive intracellular Ca2+ stores.

During growth, the retina analyzes the projected image to achieve a close match between eye length and focal length.
Since the messengers that are released by retina and choroid and that regulate eye growth are largely unknown, we screened for genes that are differently expressed in response to changes in the retinal image using the Differential Display Polymerase Chain Reaction method (DD-RT-PCR). Screening about 40-50% of the retinal and choroidal mRNA`s we found 12 genes in the retina and 5 genes in the choroid whose expression was affected by refractive changes within one day (Feldkaemper et al., Invest. Ophthalmol. Vis. Sci. 2000;41:1623-1628).

Only one out of 10 sequenced products could be identified as cytochrome-c-oxidase, subunit I.
Northern blot analysis confirmed its 2-fold up-regulation after positive lens wear and also changes of four other investigated unknown genes. Some of these genes changed upon exposure to defocus in a sign specific fashion. This requires complex and yet unknown image processing.

A directionally cloned cDNA library was constructed using the superscript plasmid system for cDNA synthesis.
For this library retinal tissue from 8 to 21 days old chicks (White leghorn) was used. After size selection, cDNA inserts were cloned into the NotI/MluI site of the pSPORT1 vector. The average insert size was about 1 kB. Expressed sequence tags (ESTs) were analyzed with the GenBank BLASTN program to identify sequence homologies to known genes.
The cDNA library was constructed to generate a profile of genes expressed in the native chicken retina and to identify novel, uncharacterized genes. Moreover, to obtain sequence information that shall be used to identify genes whose differential expression during myopia development has already been found using a differential display screen.
A detailed description of the library was presented at the 9th International myopia conference, Hong Kong 2002.

During the last years, photorefraction was applied in a wide variety of animals (Frank Schaeffel), including mice.

Because the genome of the mouse is widely known, we plan to investigate gene expression and search for genes that are controlled by visual input using commercially available DNA chips.

Recently, we found that intravitreal insulin has a dramatic effect on eye growth in chickens. Even with positive lenses, which normally induce hyperopia, high amounts of myopia are produced within a few days. The myopia is due to axial elongation, but the major growth occurs in the anterior chamber depth and the crystalline lens. Interestingly, the effects of insulin are most prominent when either positive or negative lenses are worn - there is only a weak effect when the visual experience of the animals was normal.

Insulin injections also dramatically increased ZENK mRNA levels and ZENK protein expression in the retina.

The manuscript of this study is in revision for IOVS (2008).